1. Field of the Invention
The present invention relates to a printing apparatus, particularly to a technique for driving a mechanism for feeding or conveying a printing medium at a high speed and a high accuracy.
2. Description of the Related Art
There have been various proposals for facilitating the conveyance accuracy of the printing medium in the printing apparatus for forming an image by printing means while conveying the printing medium through the interior of the printing apparatus (for example, see Japanese Patent Application Laid-open No. 2002-28313). Recently, means for detecting a present position of the printing medium and controlling the conveying speed of the printing medium by using the detected content becomes an indispensable component for forming the image at an aimed position on the printing medium (for example, see Japanese Patent Application Laid-open No. 2002-137469). A conventional method for controlling the conveyance of the printing medium will be described below.
FIG. 1 is a schematic view for explaining a main part of a conveyor system in the prior art printing apparatus. In the drawing, reference numeral 1001 denotes a first conveyor roller, and 1002 denotes a second conveyor roller. Also, reference numeral 1003 denotes a first pinch roller corresponding to the first conveyor roller 1001, and 1004 denotes a second pinch roller corresponding to the second conveyor roller 1002. The conveyor rollers 1001 and 1002 convey a printing medium 1007 in the direction indicated by an arrow, while nipping the printing medium 1007 between them and pinch rollers 1003 and 1004 thereof, respectively. Reference numeral 1008 denotes a conveyor motor. The conveyor rollers 1001 and 1002 are made to rotate by the engagement thereof with a drive shaft of the conveyor motor 1008. An image is printed in an area of the conveyed printing medium 1007 between the two conveyor rollers 1001 and 1002 by a head cartridge 1 mounted to a carriage 2.
A rotational angle sensor 1006 and a code wheel 1005 constitute means for detecting a conveying distance and a conveying speed of the printing medium. The code wheel 1005 is fixed to a rotary shaft of the first conveyor roller 1001. While, the code wheel 1005 is provided with slits cut at a constant pitch in the circumferential end portion thereof. A position of the respective slit can be detected by the rotational angle sensor 1006 fixedly disposed within the printing apparatus.
FIG. 2 is an enlarged view illustrating the detection of the slits 201 on the code wheel 1005 by the rotational angle sensor 1006. The slits 201 are cut on the code wheel 1005 at a constant pitch. The rotational angle sensor 1006 is a transparent type optical sensor for detecting the moving slit 201 and issuing a pulse signal at a timing of detection. The rotational angle of the code wheel 1005 is detected by the issued pulse signal. The position, speed and acceleration of the printing medium 1007 are calculated by the rotational angle, the time interval for issuing the pulse signals, or others. Further, by using the value thus obtained, it is possible to control the rotational speed or others of the conveyor roller 1001.
FIG. 3 is an illustration for explaining a conventional profile for controlling the conveyance of the printing medium. In the drawing, an abscissa axis represents a time passage. Curve of a represents a distance from the detected point of the printing medium to a target point. And curve of b represents the conveying speed of the printing medium. In general, as illustrated, when the position of the printing medium is far from the target point, the conveying speed is accelerated for a predetermined period, maintained at a constant speed, and then decelerated when approaching to the target point. Finally, the printing medium is controlled to stop at the target point.
In the conventional printing apparatus, the conveyance of the printing medium is controlled as described above. When it is necessary to control the conveyance of the printing medium at a higher speed and a higher accuracy, the technique has been improved to facilitate the accuracy of the mechanical dimension of the first conveyor roller for conveying the printing medium and to control the rotational angle of the first conveyor roller at a higher speed and a higher accuracy.
Recently, however, the requirement has been more complicated, for example, when a high grade image having a photographic image quality is printed by using ink droplets of a micro-size ejected at a higher density and a higher accuracy. Under such a circumstances, it is necessary to rapidly improve the conveyance accuracy of the printing medium, whereby there has been a limit in the conventional mechanism and the prior art control method.
For example, if the conveying speed is extremely decelerated, there is a problem in that an output from the rotational angle sensor 1006 becomes discrete to make the speed control to be very difficult. Concretely, by the speed deceleration, a mechanical frictional load or others may vary whereby a pulse signal necessary for obtaining an actual speed becomes discrete while containing errors. Accordingly, the speed control carried out in accordance with this pulse signal is liable to be unstable. To avoid this problem, there is a method for facilitating the resolution of the slit 201 in the code wheel 1005. However, this is limitative in practice in the manufacture of the code wheel. As an alternative method for facilitating the resolution, a diameter; i.e., a circumferential length; of the code wheel may be enlarged to increase the number of slits. This method, however, is problematic because a size of the printing apparatus itself becomes larger.
Also, the eccentricity during the attachment of the code wheel 1005 and the conveyor roller 1001 is problematic, and not negligible when the control is more precisely carried out.
In addition, there is another problem in the accuracy of the stop position. If it is required to stop the printing medium at the accuracy higher than the resolution of the code wheel 1005, a true position could not be known between the adjacent two slits. Thereby the printing medium is made to stop based on the presumption. In such a case, there is a risk in that the stop position may fluctuate relative to the presumed position due to the variation of the mechanical frictional load or others.
Further, the conveying distance obtained by the conventional system is a calculated value indirectly obtained from the rotational angle of the code wheel 1005, which is not directly resulted from the measured distance of the printing medium 1007. Accordingly, all errors becomes the error of the conveying distance, such as a dimensional error or attachment error of the parts disposed downstream from the code wheel 1005 or a slippage due to the difference in friction between the printing medium 1007 and the pinch roller.
Drawbacks will be described below, which may occur when the prior art method having the above-mentioned problems is used for controlling the printing medium under the recent circumstances.
1) Recently, there are various printing media or others to be conveyed, such as a plain paper, a coated paper, a glossy paper or a plastic tray for the CD-R printing. Accordingly, a surface property of the printing medium or an object to be conveyed, such as a coefficient of friction may be widely changed to result in various frictional forces between the object to be conveyed and the conveying roller. Thus, the actual conveying distance is variable relative to the same rotational angle of the conveyor roller in accordance with kinds of the conveyed object, whereby there is a problem in that the accurate conveying distance is not obtainable by solely controlling the rotational angle of the conveyor roller.
2) A gear or an encoder used for controlling a gear driving the conveyor system has a slight eccentricity or deflection. Thereby, the actual conveying distance more or less contains an error of the above-mentioned mechanical system even if the rotational angle is correctly controlled by using the sensor. This error is not negligible in the high conveying accuracy required for the recent printing apparatus.
3) In the conventional system for converting the value obtained from the rotational angle sensor to the conveying distance of the printing medium, a diameter of the wheel which is a scale of the rotational angle sensor may be increased for the purpose of further enhancing the resolution of the detectable rotational angle. In this case, however, since a size of the wheel is directly related to a size of the printing apparatus, the enlargement of the wheel size must be naturally limited under the recent circumstances in which the minimization of an apparatus size is important. Accordingly, there is also a limitation in the improvement in the resolution of the rotational angle; i.e., the conveyance accuracy.
4) Nowadays, the requirement for a so-called full-bleed printing has increased, in which the printing is carried out until reaching the endmost edge of the printing medium. When the endmost edge of the printing medium is printed in this system, there is a stage in which the printing medium is left from the first conveyor roller and is conveyed solely by the second conveyor roller. A slight error in the conveyance performance inevitably exists between the first and second conveyor rollers due to the difference in the mechanical transmission passage. This problematic in that such an error results in the shift of the printing position in the printing of the rear end portion and causes a significant drawback of the image. In the recent full-bleed printing, a countermeasure therefor is adopted by minimizing a length of the printing medium to be once conveyed to suppress the mechanical error. However, such a countermeasure causes a novel problem in that the printing speed becomes lower.